Urethanes have been used in the manufacture of solid tires useful for such applications as industrial tires, off-the-road tires, bicycles tires and the like. They have not been entirely satisfactory in such applications because such urethane solid tires do not have the proper cushioning and handling characteristics for a soft vehicle ride on such applications as passenger vehicles. Also, such solid tires suffer from internal heat build-up and subsequent degradation of the elastomer material in prolonged high speed service conditions or under rough terrain situations where the tire is being deformed.
Various polyurethane elastomers have been proposed for use on such solid tires, as described in U.S. Pat. No. 4,934,425, owned by the same assignee, in which there is provided a non-pneumatic tire rotatable about an axis, having improved hysteresis and flex fatigue resistance.
The invention described above and taught in U.S. Pat. No. 4,934,425 was based on the use of a polyether polyol prepolymer having a terminal functional group containing active hydrogen capable of reacting with an isocyanate group. For commercial application, prepolymers using toluene diisocyanate are manufactured. These prepolymers have come into recent disfavor from an environmental and safety standpoint, because TDI is volatile and thus requires special handling procedures. Another drawback to the use of TDI is that free TDI remains in the prepolymer and unless extracted, it is too reactive for use with many common chain extenders except those with very low reactivities.
Attempts to use lower levels of TDI yet maintain the high flex fatigue resistance properties have to date been unsuccessful. Prepolymers prepared according to the teachings of G.B. 1,101,410 wherein the TDI monomer content has been reduced to less than 0.5% have greatly reduced flex fatigue properties. Such materials would not have the flex fatigue properties useful in non-pneumatic tire and other dynamic flex applications. When using the teachings of U.S. Pat. No. 4,934,425 and reducing the TDI content below 0.5%, again greatly reduced flex fatigue properties result.
It is a further object of this invention to provide a urethane elastomer with flex fatigue properties equal to or better than those obtained when the prepolymer is formed by reacting binary polyols with a traditional isocyanate, with a corresponding level of free isocyanate.
It is a still further object of this invention to insure that the urethane so cured will be suitable for use in a non-pneumatic tire.
This invention surprisingly demonstrates the criticality of utilizing a specialized trimodal molecular weight urethane prepared from a prepolymer having a reduced free TDI monomer content with three distinct molecular weight peaks. These peaks and low residual isocyanate are both necessary to achieve high flex fatigue resistance. The prepolymer is cured preferably with an organic diamine curative to yield a polyurethane elastomer providing the balance in properties required for good vehicle ride characteristics as well as long life.